This study is a case-control study of incident childhood leukemia (all subtypes) diagnosed since mid-1995. Children newly diagnosed with leukemia are enrolled in the study. Criteria for inclusion in the study are: under 15 years of age, no prior cancer diagnosis, residency in the state of California at the time of diagnosis, and availability of an English or Spanish speaking parent or guardian. Pre-treatment biological specimens, including bone marrow and peripheral blood, are obtained for analysis in the UCB lab of Dr M. Smith. The lab will use Fluorescence In Situ Hybridization (FISH) to detect chromosome specific aneuploidy and translocations. A number of chromosomal translocations, including t(9;22) and t(8;21), are known to be centrally involved in the development of childhood leukemia. Molecular characterization of the cases with translocations may provide insight into the timing of critical exposures and the nature of the etiological agent involved.